A221651 Numbers divisible by their first digit squared (excluding those whose first digit is 1).

20, 24, 28, 36, 48, 50, 200, 204, 208, 212, 216, 220, 224, 228, 232, 236, 240, 244, 248, 252, 256, 260, 264, 268, 272, 276, 280, 284, 288, 292, 296, 306, 315, 324, 333, 342, 351, 360, 369, 378, 387, 396, 400, 416, 432, 448, 464, 480, 496, 500, 525, 550, 575

Offset

1,1

Comments

Numbers where floor(n/10^floor(log(n)))^2 divides n.

Links

Christian N. K. Anderson, Table of n, a(n) for n = 1..10000

Christian N. K. Anderson, Ulam Spiral for the first 10000 terms. [Draw the usual Ulam spiral, containing all the nonnegative integers, and color the numbers belonging to this sequence yellow]

Example

48 is divisible by 4^2.

Prog

(R)
x=0; y=rep(0, 1000); len=0
firstdig<-function(x) as.numeric(substr(as.character(x), 1, 1))
isint<-function(x) x==as.integer(x)
while(len<10000) if((fd=firstdig((x=x+1)))>1) if(isint(x/fd^2)) y[(len=len+1)]=x

Crossrefs

Cf. A132359, A034709, A034837, A005349, A007602, A034838, A225297, A225722, A225683.

Sequence in context: A250289 A114843 A174629 * A108373 A109131 A171696

Adjacent sequences: A221648 A221649 A221650 * A221652 A221653 A221654

Keyword

nonn,base

Author

Kevin L. Schwartz and Christian N. K. Anderson, May 04 2013

Status

approved